The United States is sending 200 additional military personnel to Syria to help the campaign to drive the Islamic State militant group from Raqqa, U.S. Defense Secretary Ash Carter said on Saturday.

Speaking in Bahrain at the Manama Dialogue conference on Middle East security, Carter said the 200, including special forces trainers, advisers and explosive ordnance disposal teams, would join 300 U.S. special force troops already in Syria.

"These uniquely skilled operators will join the 300 U.S. special operations forces already in Syria, to continue organizing, training, equipping, and otherwise enabling capable, motivated, local forces to take the fight to ISIL," he said in a speech, referring to Islamic State.

The first goal of a coalition opposed to the militants was to "destroy the ISIL cancer’s parent tumor in Iraq and Syria, because the sooner we crush both the fact and the idea of an Islamic state based on ISIL’s barbaric ideology, the safer we’ll all be", he said.

He added that Russia, President Bashar al-Assad's top foreign military backer, had "only inflamed the civil war and prolonged suffering of the Syrian people."

Syria's civil war pits Assad, backed by Iran, Russia and some Shi'ite militias, against mostly Sunni Arab rebels backed by Turkey, Gulf monarchies and the United States. A secondary conflict puts all of them at war with Islamic State, an effort that coincides with a drive against the group in Iraq.

The Iraqi city of Mosul and the smaller Syrian city of Raqqa are the two pillars of Islamic State's self-declared caliphate, and recapturing them would be a pivotal defeat for the ultra-hardline Sunni jihadists.